Method of burning boron containing fuels in a jet engine to minimize boron oxide deposits



June 6, 1961 E BECKER 2,986,876

METHOD OF BURNING BORON CONTAINING FUELS IN A JET ENGINE TO MINIMIZEBORON OXIDE DEPOSITS Filed July 13, 1959 IN V EN TOR fa G/VE flm GKATTORNEY 2,986,876 METHOD OF BURNING BORON CONTAINING FUELS IN A JETENGINE T MINIMIZE BORON OXIDE DEPOSITS Eugene Becker, Grand Island,N.Y., assignor to Olin h iathieson Chemical Corporation, a corporationof Filed July 13, 1959, Ser. No. 826,879 3 Claims. (Cl. 6035.4)

This invention relates to a method of minimizing boron oxide deposits onsurfaces within jet-type aircraft engines operating on a boroncontaining fuel. By boron containing fuel is meant a high-energy fuelsuch as a boron hydride, including diborane, the pentaboranes, anddecaborane, organoboranes, including the lower alkyl pentaboranes andlower alkyl decaboranes, and conventional hydrocarbon jet fuelscontaining the same. Lower alkyl pentaboranes can be prepared, forexample, according to the method described in application Serial No.546,803, filed November 14, 1955, now abandoned, of Jack R. Gould andJohn E. Paustian. Lower alkyl decaboranes can be prepared, for example,according to the method described in application Serial No. 557,634,filed January 6, 1956 to Joseph A. Neif and Edward J. Wandel.

The present invention can be applied to any of the three basic types ofjet engines, i.e. the ram-jet, the turbojet, and the turbo-prop,although it has a particular application to the turbo-jet and turbo-proptype engines, and will be further described as applied to a turbo-jetengine.

In the operation of a turbo-jet aircraft engine, air flows into theengine through the air entrance section and then into the compressorsection, where it is usually compressed to a pressure of about 45 to 180p.s.i.g. The air entrance and compressor section may follow any one ofseveral designs; and the compressor section may possess either an axialor a centrifugal compressor. If of the centrifugal type, the compressormay additionally possess either a single or a double entry.

From the compressor section, the compressed air flows into thecombustion section where it is combined with a metered and atomized orprevaporized amount of fuel and its temperature increased by combustionof the fuel. It will be noted that the air flow in this section is suchthat only a relatively small amount of the air actually mixes with thefuel at the point of combustion. This portion of the air is generallyreferred to as the primary air supply. The weight ratio of primary airto fuel is generally between 10 to 1 and 50 to 1.

Following the combustion of the fuel, the combustion products are almostimmediately and intimately mixed with the remaining or secondary air.Thus, the combustion products are cooled from a combustion temperatureof about 3500" to 4000 F. to an average temperature of about 1400 F. Thelatter temperature is dietated at the present time by the types ofmetals and metal alloys that are presently available for use within theturbine section of an engine.

The gaseous products of combustion and the excess air entering theturbine section from the combustion section cause the turbine rotor orrotors to revolve and to drive the compressor in the compression sectionand also auxiliary equipment such as fuel pumps, lube oil pumps,generators, etc.

The gases leaving the turbine then flow into the tailpipe section fromwhence they vent to the atmosphere. The design of this section may varyconsiderably. For example, it may have either a single or double exit,and it may also be of the variable orifice or adjustable exhaust nozzletype. When the tailpipe is provided with an Afterburner, it is essentialthat a variable exhaust 36,76 Patented June d, 1961 opening be providedto adjust for both normal and afterburning combustion conditions. Thetailpipe section normally operates at temperatures of about 900 to 1400F.

When afterburner nozzles are provided in the tailpipe section, adiffuser is usually placed between the turbine section and the nozzles.This device serves to redistribute the gas flow in the tailpipe and topromote better combustion of the fuel issuing from the afterburnernozzles.

The combustion section may be any one of the conventional types as, forexample, one that employs multiple combustion chambers (cans) or onethat uses an annular combustion liner or chamber (a burner basket). Inthe first of these types, the air flow is split upon leaving thecompressor and equal portions sent to each can, where these portions arecombusted with portions of the fuel. The combustion products are thenrecombined with secondary air and routed to the turbine section.

When the combustion section is of the burner basket type, the primaryportion of the air is diverted from the main stream and directed towardthe fuel injector within the basket where it burns with the fuel. Theremaining or secondary air is then mixed with the products of combustionat a point prior to their entrance into the turbine section.

The turbine section of a jet engine may contain one or more turbinerotors and one or more stages. In addition, the turbine blades may be ofthe impulse and/or reaction types and may or may not be shrouded.Associated with the turbine rotor blades are stator blades which directthe hot gases against the rotor blades.

One of the more serious problems associated with the use of boroncontaining fuels in turbo-jet aircraft engines results from theformation of boron oxide as a combustion product and its subsequentdeposition on the surfaces of the combustion section, the turbinesection, and the tailpipe section. For example, a pound of diboraneproduces 2.5 pounds of B 0 and a pound of pentaborane, 2.76 pounds. Theglass-like boron oxide has an approximate melting point of 840 F. andhas a high viscosity at turbine operating temperatures of about l400 to1600 F. Thus boron oxide deposits collect and flow along engine parts,including combustor walls and transition pieces, turbine rotor andstator blades, tailpipe walls, including afterburner parts, andvariablearea nozzles.

It has been proposed to minimize deposits of boron oxide in turbo-jetengines by avoiding the strong reverse flow that is usually designedinto a high velocity combustor to provide flame piloting. The wideflammability limits and high flame speeds of boron fuels make itpossible to reduce the piloting otherwise required by hydrocarbon fuels.It has also been proposed to minimize boron oxide deposits by filmingthe combustor walls with air to prevent impingement of boron oxide onthe surfaces thereof. Various methods of providing the air film areavailable such as porous walls, lonvers, step construction, etc. Theseproposals, however, have not been successful in alleviating the boronoxide deposition problem.

In accordance 'with the present invention, boron oxide deposits areminimized within the combustion, turbine and exhaust sections of anair-breathing jet-type aircraft engine operating on a boron containingfuel by introducing a normally liquid ether of silicon wherein the ratioof silicon to oxygen is within the range of 0.5 to 4 within theappropriate section.

The silicon ether can be introduced within those sections where boronoxide is prone to form by any one or more of several known meansincluding introduction within the air or combustion gas stream flowingto the pertinent sections and introduction directly into the pertinentsections. The silicon ether can be introduced within the combustionsection, for example, by incorporation into the primary or secondary airstream from the compressor section. Thesilico'ntether can.be introducedwithin the turbine section, for example, by direct injection as byutilizing a porous material, such as a ceramic material or sinteredstainless steel, for construction of the turbine stator blades andtranspiring the silicon ether through the stator blades. The siliconether can be introduced within the exhaust section byv direct injectioninto the gas stream flowing therein. Where an afterburner is employed,the silicon ether can be. injected at or subsequent to the diflfuser andalso in the air stream supplied to the afterburner.

In order to test the efficiency of silicon ethers in minimizing thedeposit of boric oxide on heated surfaces, a combustion and exhaustsystem similating that of a jettype engine was constructed and is showninthe attached drawing wherein FIGURE 1 isa side elevation andVFIG- URE2 is an isometric of the combustion and exhaust system, FIGURE 3 is asection along line AA of FIG- URE 1, FIGURE 4 is a section along lineB--B of FIGURE 1, FIGURE 6 is a section along line CC of FIGURE 1 andFIGURE 5 is a section along line D--D of FIGURE 4. I r

In the drawing numeral 1 represents a section of four inch diameterstainless steel pipe about sixty-eight inches long and divided into foursections by flanges 2, 3, 4 and 5. Attached to pipe 1 by means of flange6 was an eight inch diameter section of stainless steel pipe aboutinches long defining the exhaust chamber.

The section of pipe 1' between flanges 2 and 3 had disposed therein anInconel liner 8, defining the combustor, with an average diameter of3.12 inches which tapered outwardly toward flange 3 one-eighth inch perfoot and was two feet long. Liner 8 was perforated and contained eightyholes varying from three-sixteenths inch to three-eighths inch indiameter. Air entered the combustor through inlets 9 and 9a. The bulk ofthe air entered inlet 9 at 160 F. JP'4 fuelwas injected into thecombustor by means of inlet 10 and boron containing fuel was injectedinto the combustor by means of inlet 11. Inlet 11 was connected to astandard 80 hollow cone oil burner nozzle and inlets 9a and 10 wereconnected to a fuel-air nozzle which injected the mixture into thecombustor by means of an annular opening around the burner nozzle. The'fuel-air mixture was ignited by means of a high energy, high voltage,air-gap type spark plug 12. r g

The section of pipe 1 between flanges 3 and 4 was fourteen inches long.Two inches'upstre'am'from flange. 4 was thermocouple bank 13 consistingof two thermocouples. r

The section of pipe 1 between flanges 4 and S was ten inches and definedthe test section, which contained three Vycor-glass Windows and a probeportlocated 'circumferentially around pipe 1 midwayin the. test section.Through the probe port was inserted deposition probe 14 which consistedof a section of'one half' inch stainless steel tubing with the baseclosed by welding. The leading edge had a projected deposition area of1.5 square inches. Probe 14 was attached to pipe" 1 by a stainless steelflare. Just upstream of and parallel to probe 14 was silicon etherinletlS communicatingwith reservoir 16. Nitrogen pressure, usually 0.5to 1.0 p.s.i., was. applied to reservoir 16 by means of linev 17. Theflow rate of the silicon ether was controlled by varying the nitrogenpressure applied to the reservoir. V

The section of pipe 1 between flanges 5 and .6 was ten inches long; Sixand one-half inches downstream from flange '5 was thermocouple bank 18consisting of eight thermocouples. 1 This combustion and exhaust. systemwas. employed in each of the following examples. r

Example 1 Air flow through inlets 9 and 9a is established and JP-4 isinjected through inlet 10 and burned in the combustor at the flowraterequired to provide an exhaust gas temperature of 1500 F. measuredby thermocouple bank 18. Operation with JP-4 alone is continued for apreheating period of five minutes. A mixture of trimethoxyboroxine andacetone (4 parts trimethoxyboroxine and 1 part acetone) is then injectedinto the combustor through inlet 11 at a flow rate of 0.0055 pound persecond for iutervalsof threc, four or five minutes, the JP-4 flow ratebeing adjusted to maintain a temperature of 1500 F. at thermocouple bank18. At the end of the three, four and five minute intervals oftrimethoxyboroxine flow, JP-4 combustion is continued for thirty secondsat which time the deposition probe 14 is removed and weighed. By thismeans it is determined that the boric oxide deposition rate on the probeis 0.06 pound per hour.

The probe is cleaned and replaced and the system again brought topreheat temperature with JP-4. A Dow-Corning dimethyl silicone fluidhaving a viscosity at 25 C. of centistokes is then charged to reservoir16 and nitrogen pressure is: applied until a flow of silicone fluidthrough inlet 15 is established sufficient to wet the leading edge ofthe probe as observed through the Vycor glass windows. Thetrimethoxyboroxine-acetonc flow is then started, the IP-4- flowratebeing adjusted to maintain a temperature of 1500 F. at thermocouplebank 18. The experiment is continued for about 4.5 minutes until allofthe silicone fluid has been expended. As observed through the Vycorglass windows, the silicone fluid is efiective in preventing boron oxidedeposition on the portions of the probe wetted by the silicone fluid.

The test conditions are summarized as follows:

Silicon-e fluid-flow 44 grams/min.

' Example. 11

' This experiment was carried out in a manner similar to Example I.except that tetraethoxysilane is employed instead of the silicone fluid.As observed through the Vycor'glasswindows, the tetraethoxysilane isalso efiectivev in preventing ,boronroxide deposition on the probe.

In place of the tetraethoxysilane employed in Example II, other loweralkoxysilanes can be employed such as tetramethoxysilane,dimethyldiethoxysilane, ethyltriethoxysilane, amyltriethoxysilane, andvinyltriethoxysilane. The hydrolysis and condensation or polymerizationproducts obtained by the controlled hydrolysis of the alkoxysilanes oraryloxysilanes with water can also be employed. Note US. Patent2,416,503 and 2,416,504 to Trautman et al. for a description of thesematerials.

In place of the silicone fluid employed in Example 1, other siliconefluids of lower and highermolecular'w'eight can be employed, such asthose described -as the first type in US. Patent 2,765,221 to Luscbrinket al., including hexamethyldisiloxane a. viscosity of 0.65 centistokeat 25 C. to polysiloxanes. having a viscosity as high as 60,000centistokes or more at 25 C. These silicone fluids aregenerally producedby condensing, dialkylsilanediols or diarylsilanediols,;or.mixturesthereof, wherein the alkyl and aryl groups contain 1' to 7 carbon.atoms, and then endblocking the .polymer by admixing therewith atrialkylor triarylsilanol. .Also. the cross-linked polymers obtained,for example, by ..condensing .amixtureof organo-silane triols with'organo-silanediols and organo- 'silanols orboth, as described in abovementioned U.S

Patents -2,416,503; 2,416,504 anda2,765-,221. can beem ployed.

The amount of silicon ether required to be introduced within the varioussections of the engine in order to minimize the deposition of boricoxide varies with the characteristics of the engine and the conditionsunder which it operates. For example, in ram-jets, where a lesserdeposit problem exists than in turbo-jets, less silicon ether isrequired. In general, however, the total amount of silicon etherintroduced within the various engine sections is sufiicient to maintainthe deposition of boric oxide desirably low.

I claim:

1. A method of minimizing boron oxide deposits within the combustion,turbine and exhaust sections of an air-breathing jet-type aircraftengine operating on a fuel containing at least one material selectedfrom the class consisting of boranes and organoboranes which consistsReferences Cited in the file of this patent Proell et al., The Journalof Space Flight, vol 2, No. 1, Jan. 1950, pages 1-9, at page 2. (Copy inScientific LiyJ

1. A METHOD OF MINIMIZING BORON OXIDE DEPOSITS WITHIN THE COMBUSTION,TURBINE AND EXHAUST SECTIONS OF AN AIR-BREATHING JET-TYPE AIRCRAFTENGINE OPERATING ON A FUEL CONTAINING AT LEAST ONE MATERIAL SELECTEDFROM THE CLASS CONSISTING OF BORANES AND ORGANOBORANES WHICH CONSISTS OFINTRODUCING WITHIN SUCH A SECTION IN CONTACT WITH SURFACES UPON WHICHBORON OXIDE WOULD NORMALLY DEPOSIT A NORMALLY LIQUID ETHER OF SILICONWHEREIN THE RATIO OF SILICON TO OXYGEN IS WITHIN THE RANGE OF 0.5 TO 4IN AN AMOUNT SUFFICIENT TO MINIMIZE BORON OXIDE DEPOSITION.